Actuators producing mechanical movement of an object in response to the application of a stimulus, such as electrical power, are well known. Among the types of actuators that are small and that respond to the application of electrical power to produce a linear motion are electrothermal actuators. Examples of such actuators are described in U.S. Pat. Nos. 4,759,189 and 4,887,429, which are incorporated by reference. Within these electrothermal actuators, a closed chamber contains a working fluid. The working fluid is mostly a liquid at ambient temperature and evaporates when heated. That gas phase of the fluid expands upon heating, increasing internal pressure within the chamber. In the following description, the reference to a fluid encompasses both of the liquid and gas phases, the gas phase expanding upon heating to provide the motive force of the actuator. The chamber includes an electrically powered heater that supplies heat to the fluid, in response to an electrical current supplied to the heater. The heat produces the phase change in the working fluid and pressure increase within the chamber. In response to the increased internal pressure in the chamber, a flexible rolling diaphragm, usually peripherally clamped to the package of the electrothermal actuator, is displaced. The diaphragm displacement pushes a piston that drives a piston rod in a linear direction, usually to increase the protrusion of the piston rod from the package of the electrothermal actuator.
Typically, an electrothermal actuator includes a return spring biasing the piston to withdraw the piston rod into the package of the actuator. The expansion of the working fluid and the movement of the diaphragm provide a force that counteracts the restoring force of the spring. In order to maintain a particular extension of the piston rod in many conventional electrothermal actuators, the pressure within the chamber must be sustained by continuing the flow of electrical current to the heater. In some applications, the continuing consumption of electrical power to keep the piston rod extended is undesirable and at least a reduction in the amount of electrical power supplied to maintain the extension of the piston rod is, in many applications, highly desirable
One known electrothermal actuator, described in U.S. Pat. No. 5,396,770, reduces, and even eliminates, the necessity for continuing the supply of electrical power after extension of the piston rod. The electrothermal actuator described in that patent includes a latch mechanism that engages the piston rod when the piston rod is fully extended. The latch prevents retraction of the piston rod so that electrical power flow to the heater can be terminated. In order to release the piston rod so that it retracts within the package of the electrothermal actuator, the latch mechanism must be released. The latch mechanism includes a coil and a magnetic member. A magnetic field is produced by applying an electrical current to the coil. The magnetic field moves the magnetic member, releasing the latch, so that the piston rod retracts into the package of electrothermal actuator. When the piston rod is latched, the piston rod does not return to the initial position, with the piston rod retracted, upon loss or withdrawal of electrical power. In other words, the fail-safe position of the piston rod depends upon whether the piston rod has been latched before the loss of power. In many applications it is desirable that the fail-safe position, as in actuators without latches, be the retracted position of the piston rod.